Prevent use of unaligned uint16_t pointers

UBSan on clang complains about dereferencing unaligned uint16_t
pointers even when the dereference is being done by memmove().
Strictly it's correct as even setting the pointer to such a
value is undefined behaviour according to the standard.

Silence the noise by changing the pointers to uint8_t and
adjusting the arithmetic on them as necessary.

Some bounds checks to ensure fast code won't read beyond the
end of its input are also adjusted to prevent any possibility
of generating an address beyond the limits of the input memory.

Author: daviesrob

htscodecs/rANS_static32x16pr.c

@@ -165,7 +165,6 @@ unsigned char *rans_compress_O0_32x16(unsigned char *in,
     } else {
         // Branchless version optimises poorly with gcc unless we have
         // AVX2 capability, so have a custom rewrite of it.
-        uint16_t* ptr16 = (uint16_t *)ptr;
         for (i=(in_size &~(NX-1)); likely(i>0); i-=NX) {
             // Unrolled copy of below, because gcc doesn't optimise this
             // well in the original form.
@@ -197,15 +196,15 @@ unsigned char *rans_compress_O0_32x16(unsigned char *in,
                 int c1  = rp[3-1] > sy[1]->x_max;
 
 #ifdef HTSCODECS_LITTLE_ENDIAN
-                ptr16[-1] = rp[3-0]; ptr16 -= c0;
-                ptr16[-1] = rp[3-1]; ptr16 -= c1;
+                memcpy(&ptr[-2], &rp[3-0], 2); ptr -= c0 * 2;
+                memcpy(&ptr[-2], &rp[3-1], 2); ptr -= c1 * 2;
 #else
-                ((uint8_t *)&ptr16[-1])[0] = rp[3-0];
-                ((uint8_t *)&ptr16[-1])[1] = rp[3-0]>>8;
-                ptr16 -= c0;
-                ((uint8_t *)&ptr16[-1])[0] = rp[3-1];
-                ((uint8_t *)&ptr16[-1])[1] = rp[3-1]>>8;
-                ptr16 -= c1;
+                ptr[-2] = rp[3-0];
+                ptr[-1] = rp[3-0]>>8;
+                ptr -= c0 * 2;
+                ptr[-2] = rp[3-1];
+                ptr[-1] = rp[3-1]>>8;
+                ptr -= c1 * 2;
 #endif
 
                 rp[3-0] = c0 ? rp[3-0]>>16 : rp[3-0];
@@ -217,15 +216,15 @@ unsigned char *rans_compress_O0_32x16(unsigned char *in,
                 int c2  = rp[3-2] > sy[2]->x_max;
                 int c3  = rp[3-3] > sy[3]->x_max;
 #ifdef HTSCODECS_LITTLE_ENDIAN
-                ptr16[-1] = rp[3-2]; ptr16 -= c2;
-                ptr16[-1] = rp[3-3]; ptr16 -= c3;
+                memcpy(&ptr[-2], &rp[3-2], 2); ptr -= c2 * 2;
+                memcpy(&ptr[-2], &rp[3-3], 2); ptr -= c3 * 2;
 #else
-                ((uint8_t *)&ptr16[-1])[0] = rp[3-2];
-                ((uint8_t *)&ptr16[-1])[1] = rp[3-2]>>8;
-                ptr16 -= c2;
-                ((uint8_t *)&ptr16[-1])[0] = rp[3-3];
-                ((uint8_t *)&ptr16[-1])[1] = rp[3-3]>>8;
-                ptr16 -= c3;
+                ptr[-2] = rp[3-2];
+                ptr[-1] = rp[3-2]>>8;
+                ptr -= c2 * 2;
+                ptr[-2] = rp[3-3];
+                ptr[-1] = rp[3-3]>>8;
+                ptr -= c3 * 2;
 #endif
                 rp[3-2] = c2 ? rp[3-2]>>16 : rp[3-2];
                 rp[3-3] = c3 ? rp[3-3]>>16 : rp[3-3];
@@ -239,7 +238,6 @@ unsigned char *rans_compress_O0_32x16(unsigned char *in,
             }
             if (z < -1) abort();
         }
-        ptr = (uint8_t *)ptr16;
     }
     for (z = NX-1; z >= 0; z--)
         RansEncFlush(&ransN[z], &ptr);
@@ -476,7 +474,6 @@ unsigned char *rans_compress_O1_32x16(unsigned char *in,
         i32[i] = &in[iN[i]];
 
     for (; likely(i32[0] >= in); ) {
-        uint16_t *ptr16 = (uint16_t *)ptr;
         for (z = NX-1; z >= 0; z-=4) {
             RansEncSymbol *sy[4];
             int k;
@@ -490,12 +487,12 @@ unsigned char *rans_compress_O1_32x16(unsigned char *in,
             for (k = 0; k < 4; k++) {
                 int c = ransN[z-k] > sy[k]->x_max;
 #ifdef HTSCODECS_LITTLE_ENDIAN
-                ptr16[-1] = ransN[z-k];
+                memcpy(&ptr[-2], &ransN[z-k], 2);
 #else
-                ((uint8_t *)&ptr16[-1])[0] = ransN[z-k];
-                ((uint8_t *)&ptr16[-1])[1] = ransN[z-k]>>8;
+                ptr[-2] = ransN[z-k];
+                ptr[-1] = ransN[z-k]>>8;
 #endif
-                ptr16 -= c;
+                ptr -= c * 2;
                 //ransN[z-k] >>= c<<4;
                 ransN[z-k] = c ? ransN[z-k]>>16 : ransN[z-k];
             }
@@ -506,7 +503,6 @@ unsigned char *rans_compress_O1_32x16(unsigned char *in,
                 ransN[z-k] += sy[k]->bias + q*sy[k]->cmpl_freq;
             }
         }
-        ptr = (uint8_t *)ptr16;
     }
 
     for (z = NX-1; z>=0; z--)